☂️ [CQ] Manage our dependence on reflection in the plugins

Use of reflection to access hidden APIs is prone to runtime failures (see e.g., the recent #8795) and circumvents our verification baselining.

The regression reported in #8795 could have been avoided.

1. If we hadn't used reflection, we'd have seen a static compilation failure.
2. If we *had* used reflection but had it under-test, we would have detected when the API broke.

To protect against future issues, I propose a few next steps.

1. Review all the places we're using reflection and remove them where possible and add tests where not.
2. Explore a verification mechanism whereby new uses of reflection are detected (akin to our baseline verifier).

(This is an umbrella issue for the Flutter AND Dart plugins.)